1. Field of the Invention
The present invention relates to processes for treating a surface of a thermoplastic resin film which comprise subjecting the film surface to successive oxidation and coating treatments and stretching said coated film, followed by second oxidation and coating treatments, to thereby obtain a treated film which has satisfactory ink transferability and adhesion thereto and long-term stability and shows excellent printability.
The thermoplastic resin films obtained by the present invention are useful as the materials of stickers for outdoor advertisement, labels for frozen food containers, and namers (labels showing directions for use and other notes) to be applied to industrial products.
2. Discussion of the Background
Coat paper has hitherto been used as stickers for outdoor advertisement and labels to be applied to containers for frozen foods. Since coat paper has poor water resistance, the coat paper for use as the material of such stickers and labels has been further coated with a polyester film in order to enhance the water resistance thereof.
Recently, thermoplastic resin films having satisfactory water resistance, in particular polyolefin synthetic papers, are attracting attention as promising label material substitutes for the coat paper coated with a polyester film.
Such resin films are known. For details, reference may be made to, e.g., JP-B-46-40794 (the term "JP-B" as used herein means an "examined Japanese patent publication"), JP-B-49-1782, JP-A-56-118437 (the term "JP-A" as used herein means an "unexamined published Japanese patent application"), JP-A-57-12642, and JP-A-57-56224.
Because the polyolefins used as materials for such polyolefin synthetic papers are nonpolar, these synthetic papers do not always have satisfactory printability and processability. The polyolefin synthetic papers are hence usually subjected to an appropriate surface treatment before use.
Known among methods for such surface treatment is a technique in which a polyolefin film in a process for polyolefin synthetic paper production is stretched and a surface of the stretched film is subjected to an oxidation treatment and then coated with a coating agent. Corona discharge treatment, flame treatment, and the like are known as this oxidation treatment. Known coating agents for the above use include the primers (coating agents) based on polyethyleneimine or an ethyleneimine/ethylene urea copolymer as described in JP-B-40-12302, JP-B-53-6676, and JP-A-57-149363 and the cationic antistatic agents and amphoteric antistatic agents as disclosed in JP-A-50-161478, JP-B-59-27769, and JP-B-2-2910.
For example, JP-B-2-2910 discloses a water-soluble coating composition comprising: a water-soluble polymer obtained by preparing a polymer from a monomer represented by CH.sub.2.dbd.CR.sup.1 --COOANR.sup.3 R.sup.4 (wherein R.sup.1 represents a hydrogen atom or a methyl group; R.sup.3 and R.sup.4 each represents a hydrogen atom or an alkyl group having 1 or 2 carbon atoms; and A represents an alkylene group having 2 to 6 carbon atoms), a monomer represented by CH.sub.2.dbd.CR.sup.1 --COOR.sup.2 (wherein R.sup.1 is as defined above and R.sup.2 represents an alkyl group having 1 to 8 carbon atoms), and other hydrophobic vinyl monomers and quaternizing the tertiary nitrogen atoms of the polymer with a cationizing agent to make the polymer amphoteric; a water-soluble polyaminepolyamide-epichlorohydrin adduct; and a polyethyleneimine compound.
JP-A-53-8380 discloses an electrically conducting agent having an antistatic effect which comprises a polymer having repeating units represented by the following general formula (I): ##STR1##
[wherein R represents a hydrogen atom or a methyl group, R.sup.1 and R.sup.2 each represents a methyl group or an ethyl group; R.sup.3, R.sup.5 each represents a methyl group, an ethyl group, --(--CH.sub.2 --CH.sub.2 --O--).sub.m --H (wherein m is an integer of 1 to 4), or a benzyl group; R represents an alkyl group having 1 to 18 carbon atoms, an alkenyl group, or --(--CH.sub.2 --CH.sub.2 --O--).sub.p --H (wherein p is an integer of 1 to 4), X.sup.- represents a halide ion; A represents a substituted or unsubstituted alkylene group; and n represents an integer usually ranging from 10.sup.1 to 10.sup.4 ].
Such techniques in which a stretched film is subjected to an oxidation treatment and coated with a primer or an antistatic agent give prints sufficiently withstanding practical use when the film is printed immediately after the coating film formation thereon. However, if the surface-treated polyolefin resin film to be printed is one which has been stored in a high-temperature high-humidity atmosphere or one which was produced at least one year before, printing on the film especially with an ultraviolet-curing ink (UV ink) or an offset ink may result in ink transfer failures or in such poor ink adhesion that the prints obtained do not withstand practical use.
A printing film intended to have improved adhesion of ultraviolet-curing inks thereto has therefore been proposed. This film is a thermoplastic resin film with excellent printability which is obtained by coating a polyolefin resin film with an aqueous solution of an alkyl-modified ethyleneimine polymer as a primer and drying the applied solution to form a coating layer (see JP-A-1-141736).
However, the above primed printing film, when stored in a high-temperature high-humidity atmosphere, often becomes insufficient in the transferability and adhesion of ultraviolet-curing inks thereto, although it initially is excellent in these properties.
An object of the present invention is to provide a thermoplastic resin film which has antistatic properties and water resistance and is satisfactory in the transferability and adhesion of ultraviolet-curing inks thereto even after storage in a high-temperature high-humidity atmosphere.